1. Field of the Invention
The present invention relates to an apparatus for converting VGA signal to TV signal, and more particularly to a converting device for converting non-interlaced VGA signal to interlaced TV signal with scaling control.
2. Description of the Prior Art
The Video Graphics Adapter (abbreviated VGA) is a standard high-quality video adapter for known personal computers. Typically, the commercial VGA display provides a screen display resolution of such as 640 by 480 pixels. More recent VGA provides even higher display resolution of 1024 by 768 pixels in a mode known as Super VGA. Television (abbreviated TV) is a standard video receiving equipment widely used in family.
For displaying visual images on a video screen, either a non-interlaced or an interlaced scanning process with predetermined pattern of scan lines is used. Most VGA displays use non-interlaced scanning technique, while most commercial TV displays use interlaced scanning method. So, it is need to convert the VGA signal of the VGA display to TV signal first in case of intending to display the VGA signal on the TV video screen. In addition, the displaying resolution difference existing between the VGA display and the TV display is need to be considered.
The known Non-interlaced scanning process draws the entire picture every scan. That is, the scan lines sweep across the entire display surface to generate elements of a display image. FIG. 1 schematically shows how non-interlaced data is displayed on a video screen. The scan lines L0-LN are sequentially presented on the screen rapidly from left to right and relatively slowly from top to bottom. In such a displaying manner, a field constitutes a complete image frame.
FIG. 2 schematically shows how interlaced data is displayed on a display. The scan lines L0-LN are displayed on the display in an interlaced displaying manner. The interlaced displays require two scanning processes to complete each video picture. The first scan process draws the odd-numbered lines, and the second scan process draws the even-numbered lines. For example, as shown in FIG. 2, all the video data for the even numbered lines, designed by L0, L2, L4, contained in the even field is displayed first, followed by all the data for the odd-numbered lines, designed by L1, L3, L5, contained in the odd field. The even field and the odd field of the video data constitute a complete video frame. This displaying technique is known as interlaced scanning which is used in most commercial television.
From the description above, it is obvious that the scanning sequence is different between the non-interlaced scanning and interlaced scanning. The interlaced video data should be processed, stored, and converted before displaying on the TV display.
Besides, the display resolution difference between the VGA display and the TV display should be carefully processed when converting the VGA signal to TV signal suitable to be displayed on TV display. For example, the commercial VGA display available in the market provides a screen display resolution of 640 by 480 pixels or 800 by 600 pixels. The computer system operated in text mode of known DOS system provides displaying resolution of 640 pixels by 400 scan lines. On the contrary, the screen resolution of commercial televisions is fixed. For example, the display resolution in NTSC television system is 640 pixels by 448 scan lines. So, it is need to perform an enlarged or reduced process to obtain a desired TV signal suitable to be displayed on TV screen when converting the VGA signal to TV signal.
The prior art scaling method is performed based on discarding or repeating one scan line from N scan lines to achieve the enlarged or reduced purpose during scaling in vertical direction.
An example of prior art converting device for scaling interlaced video data is disclosed in U.S. Pat. No. 5,025,315, issued to Arun and Mark. The method considers each line of input data in each field sequentially, and determines whether the line is to be saved and whether the data lines in each successive field will be save, discarded, or duplicated. The prior art U.S. patent also discloses an apparatus for implementing the method for two header fields and a scaling factor. However, the prior art U.S. patent did not disclose how to convert non-interlaced video data into interlaced video data. The prior art also did not disclose how to scale the interlaced video data to a desired non-interlaced video data suitable to be displayed on TV video screen.